The present invention relates to a position measuring instrument for measuring the relative position of two relatively movable objects comprising a connecting element for connecting the scanning unit to one of the objects.
Position measuring instruments of this type are frequently used in numerically controlled machine equipment to determine the relative position of a workpiece and tool. The measuring scale is fixed relative to one of the workpiece and tool and a scanning unit is fixed relative to the other of the workpiece and tool. The scanning unit scans the measuring scale to produce a measuring signal. The attachment is such that relative movement between the workpiece and tool causes relative movement between the scanning unit and measuring scale.
A longitudinal position measuring instrument of this type is disclosed in the brochure published by Dr. Johannes Heidenhein GmbH, Traunreut, West Germany, entitled: "Encased Universal Longitudinal Measuring System ULS, Roll Opening System WMS", February 1982 edition. The instrument comprises a glass measuring scale which is scanned by a photoelectric scanning unit using transmitted light. The scanning unit is mounted in a housing and may traverse the length of the measuring scale along the measuring direction with little or no friction. The instrument also comprises a light source and four photosensitive elements. The light source is positioned on one side of the measuring scale and the four photosensitive elements, displaced relatively to one another, are positioned on the opposite side of the measuring scale. A cable conducts the scanning signals away from the photosensitive elements and supplies power to the light source. One end of the cable is connected to the scanning unit and the other end is connected to a box, which serves as a connector module, mounted on the housing.
During relative movements between the scanning unit and the housing, the distance between the scanning unit and the box is constantly changing. The changing distance means that the cable extending between the scanning unit and the box is constantly being moved. In order for these movements to take place without interfering with the measuring operation, a space is provided within the interior of the housing. However, a disadvantage arises in that the housing is required to be of increased size.
An additional disadvantage of this type of system, is that movements of the cable in a confined space within the housing may lead to damage of the cable and thereby result in failure of the measuring instrument.
A longitudinal measuring instrument is also disclosed in German Patent Application P No. 36 13 755.3 (DE-OS No. 36 13 755), which is not a prior printed publication to this invention, in which a glass graduation is scanned photoelectrically by a scanning unit fastened to a rod. The glass graduation is connected to a first object and the rod is connected to a second object, such as a machine base, movable relatively to the first object. The rod contains an axial bore through which a wire lead passes to connect the scanning unit with the outside. The wire lead protrudes out of the axial bore in an undefined manner. This results in a disadvantage in that tensile stresses may be exerted on the wire lead resulting in undesirable forces being exerted on the scanning unit.
It is an object of the present invention to provide a measuring instrument of the above-mentioned type which is compactly constructed and in which reliable performance is assured.